Non-thyroidal illness (NTI) is a syndrome where thyroid hormone levels drop in response to starvation or illnesses such as bacterial infections and myocardial infarction. The pathogenesis of NTI is incompletely understood. NTI can be modeled in animals by injection of lipopolysaccharide, as a mimic of bacterial infection. We have reported that mast cells are critical players in the pathogenesis of NTI, releasing numerous pro-inflammatory mediators in response to activation of the toll-like receptor pathway. LPS failed to induce NTI in mast cell deficient mice. Reconstituting these mice with normal mast cells restored their ability to develop NTI in response to LPS. More recently we found that leptin, in addition to mast cells, is involved in the pathogenesis of bacterial NTI. In fact, LPS failed to induce NTI in leptin knockout mice. We thus postulate in the present application the existence of a novel interaction between leptin and mast cells. Since leptin induces differentiation and activation of hematopoietic cells, and leptin KO mice has dysfunctional natural killer (NK) cells, we hypothesized leptin KO mice has dysfunctional mast cells leading non- response to inflammatory stimuli. We hypothesize the followings. In specific aim 1 we hypothesize that leptin is required for mast cell maturation. We will test the hypothesis by reconstituting leptin knockout mice with mast cells derived from wild type donors. If leptin is truly required for mast cell maturation, then leptin KO mice will acquire normal mast cells from the transfer and should now develop NTI in response to LPS. In specific aim 2 we hypothesize that leptin is required for mast cell activation at the time of NTI induction. To confirm our hypothesis, we will inject LPS plus leptin into leptin KO or mast cell deficient mice. If both are required, NTI will be observed in leptin KO mice injected LPS plus leptin. We, then, assess if mast cells (BMMC) express functional leptin receptor at protein level. If they express leptin receptor, we will assess if leptin receptor on mast cells is required inducing NTI. To assess it, we will generate BMMC from wild type control, leptin KO, and leptin receptor KO, and reconstitute them into mast cell deficient mice. If leptin receptor on mast cells is required, BMMC from leptin receptor KO mice should not restore NTI. In specific aim 3, we will assess whether the NTI mediator released from mast cells are different in the presence or absence of leptin. This grant application provides a fresh look at the pathogenesis of NTI, delineating the influence of leptin and mast cells on thyroid pathophysiology. Project Narrative: Non-thyroidal illness (NTI) is common syndrome observed in numerous human diseases. Its pathogenesis is poorly understood. In this application we propose a new mechanism for NTI based on the interaction between mast cells and leptin.